I don't think it is an issue of whether they use some tags to make it only visible by mouseover or not, I think the issue is, that you do not need to be registered at all to view it. This way _everyone_ _can_ access them and even google will reveal them in plaintext searches.
I am not sure if Talad will allow them to keep the spoilers if they made it accessible only for registered users, since it is not a guild page but a big community page. The decission, however, lays by Talad and by the admin of psde.de.
The main problem with spoilers is, that they encourage to use them rather than trying it on your own. I personally know a lot of people who do that, but also know a lot of people who first try it on their own and if they really can't find the answer to a riddle, or the correct phrase they look into a spoiler list, or ask friends for help. It has been stated often enough, that NPC's are not yet that smart and that the quest system is still far from being perfect, so it is understandable that spoilers are needed sometimes - but those who _only_ use spoilers to get all the rewards from the quests discourage the devs work. It's a lot of work to write a quest, from getting the idea for it, over making it fit into the settings to actually write the script for it.
@Integrity. I don't know who you are, but seriously, the way you have put it is pretty rude. You wanted to point out that there are public spoilers, fine, but you neither show respect for the staff of psde.de who enforce the usage of the spoiler tags [rather than allowing everyone to post spoilers without that tag], neither for the devs who will deal with it. You complain about hypocrisy, but aren't you the biggest hypocrite of all of them - by making such a blame-post but saying you do like to use the feature? Think about it - and remember it next time you make a post
They don't seem to have a problem with openly posting "SPOILERS" on their forums (using the [spoiler] tags that the forum recognizes) to all the in-game quests.
Well, I had a look at the site and it really is not like you stated it. The way you put it says they have a huge list of _all_ quests and spoilers for _each_ of them, but thats not the case. The spoilers are in their help section, where people ask for quest help when they are stuck on the phrasing, game mechanics, or can't find the answer to a riddle. I have not seen a single post there which is a pure spoiler
@Nikodemus: I agree with a lot of your points, except for one. You say people join guilds for ooc reasons to get spoilers. Well, while it is trua that most (if not all guilds) have spoilers, I doubt a player will join a guild only for that. Most (if not all) guilds recruit their members by IC means and before a member joins a guild they most probably won't know the quality and quantity of the spoilers in their forums. Btw, what is the star in your post referring to?

If everyone are running around with glyphs of doom, which are supposed to be very rare and unknown to most, is it looking realistic?
Glyph of doom? Oh gimme da spoiler!

No seriously, it's not realistic, but the problem here lays within the game mechanics, which yet reveal the same reward to every person who does the quest and in due time a lot of people will do it, with or without spoilers. A possible solution would be to change the quest system to allow random rewards and make the most rare reward have a lesser chance to be gotten than the not so rare rewards - yeah, that sounds like an idea, gonna feature request that
